By using a transmit signal precoding technology and a receive signal combination technology, a Multiple Input Multiple Output (Multiple Input Multiple Output, MIMO) wireless communications system may obtain diversity and an array gain. A system that uses precoding may be represented as follows:Y+H{circumflex over (V)}s+n; where
Y is a received signal vector, H is a channel matrix, {circumflex over (V)} is a precoding matrix, s is a transmitted symbol vector, and n is measured noise.
For optimal precoding, generally, a transmitter needs to fully learn channel state information (Channel State Information, CSI). A commonly used method is that user equipment (User Equipment, UE) quantizes transitory CSI and reports the quantized CSI to a base station, where the user equipment includes a mobile station (Mobile Station, MS), a relay (Relay), a mobile telephone (Mobile Telephone), a handset (handset), portable equipment (portable equipment), and the like, and the base station includes a NodeB (NodeB), a base station (Base station, BS), an access point (Access Point), a transmission point (Transmission Point, TP), an evolved NodeB (Evolved Node B, eNB), a relay, and the like.
CSI reported by an existing Long Term Evolution (Long Term Evolution, LTE) system includes information such as a rank indicator (Rank Indicator, RI), a precoding matrix indicator (Precoding Matrix Indicator, PMI), and a channel quality indicator (Channel Quality Indicator, CQI), where the RI and the PMI respectively indicate a quantity of used transport layers and a used precoding matrix. A set of used precoding matrices is generally referred to as a codebook, where each precoding matrix is a codeword in the codebook.
A codebook in LTE R8 is mainly designed for single user MIMO (SU-MIMO, Single User MIMO), and a precoding matrix or a codeword in the codebook meets a constraint of 8PSK (8 phase shift keying), and is quantized within a fixed range. Therefore, accuracy of space quantization is limited, and performance of a 3-dimensional MIMO (3D MIMO, 3 Dimensions MIMO) technology is greatly limited.
In an existing cellular system, a beam at a transmit end of a base station can be adjusted only in a horizontal dimension, and there is a fixed downtilt angle in a vertical dimension for each user. Therefore, various beamforming/precoding technologies and the like are based on channel information in a horizontal dimension. Actually, because a channel is 3D, a throughput of the system cannot be optimal by using a method of fixing a downtilt angle. Therefore, beam adjustment in a vertical dimension is of great significance for improving system performance.
A technical idea of the 3-dimensional MIMO technology is mainly adjusting a weight of 3-dimensional beamforming at an active antenna end according to 3D channel information estimated by a user end, so that a main lobe of a beam is “directed at” a target user in 3-dimentional space, received signal power is greatly improved, a signal to interference plus noise ratio is increased, and a throughput of an entire system is further improved.
A 3D beamforming technology needs to be based on an active antenna system (AAS). Compared with a traditional antenna, the active antenna system further provides degrees of freedom in a vertical direction.
However, in the prior art, whether for a 2D beamforming technology or a 3D beamforming technology, when a precoding matrix indicator is being fed back, precoding matrix sets used for all transmit ends and all communication scenarios are the same. Therefore, when the precoding matrix indicator is being fed back, a precoding matrix to be fed back can only be selected from same precoding matrix sets. However, in practical application, different transmit ends may be located in different communication scenarios, and a communication scenario in which a transmit end is located also varies; in this case, a problem of poor flexibility exists when a fixed precoding matrix set is used to feed back a precoding matrix indicator, and therefore, relatively poor subsequent communication quality is caused.